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SubjectRe: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields, validate user inputs
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On Jul 3, 2018, at 9:40 AM, Andi Kleen <andi@firstfloor.org> wrote:

>>
>> So I think you're good... But yes, you raise an interresting point.
>
> So it sounds like architectures that don't have an instruction atomic u64
> *_user need to disable interrupts during the access, and somehow handle that
> case when a page fault happens?

I think all this discussion of “atomic” is a huge distraction. The properties we need are:

- User code can change rseq_cs from one valid user pointer to another with a single instruction (or equivalent) such that we can’t end up in the kernel with the write only partially done as seen in that thread.

- The kernel needs to be able to read the value consistently with the above requirement.

I don’t think it’s possible to have a valid implementation of get_user() on any architecture that’s so weak that this doesn’t work.

If user code writes rseq_cs from the wrong thread, I think the user code is buggy and we simply don’t care what happens. The kernel should be allowed to use an arbitrarily weak read with respect to other threads.

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